Filtration apparatus have been used when plasticating a resin polymer material in an extrusion or mold machine having a barrel 18 with an inlet, outlet, and inner surface. More specifically, a rotatable screw 22, located within and closely spaced from the inner surface of the barrel extends from the inlet toward the outlet, and includes a core and a main flight projecting radially from the core in a helix along the axis. The rotation of the screw creates axial and radial forces of molten material as it flows downstream to the outlet.
In a typical application, solid polymer material is fed at the inlet and heated through the barrel 18 while being driven downstream under pressure by the rotating screw. A stream of homogeneously mixed, molten material is preferably produced before passing the outlet to an appropriate downstream module. The downstream module may be a die to shape the molten material into a sheet, tube or other desired extruded profile or a mold for an injection or blow molded part.
Filtration apparatus are often used to assure the quality and uniformity of molten material passing to the downstream module. Typically, the filtration apparatus includes a sliding plate having two filter assemblies A, B that contain screens as filter elements. The filtration apparatus includes an internal bore and a screen changing device 10 to move the sliding plate 16 across the bore through which the polymer material flows. To be clear, the sliding plate 16 is movable across the bore to position one filter assembly in-line, while the other filter assembly is off-line and thus accessible to cleaning or replacement. Accordingly, it is important that the filtration apparatus has a suitable sealing device to prevent the loss of molten polymer material, particularly during operation when there are substantial axial and radial forces of molten material driven by the rotation of the screw. Still further, the sealing device must be effective under relatively high operating temperatures with polymer melts having low viscosities, but yet, not interfere with the mobility of the sliding plate interchange between filter assemblies A and B.
Filtration apparatus in the past have used various sealing devices. One of the better filtration apparatus is disclosed in U.S. Pat. No. 5,507,498, issued to Delano B. Trott and titled “Sealing Device For Polymer Filtration Apparatus” (hereinafter referred to as “Trott”), the description of which is incorporated by reference. In Trott, the filtration apparatus includes a sealing device having an annular seal and a C-shaped spring. The use of the C-shaped spring results in a problem. The C-shaped spring has a gap between its ends and some molten material flowing through the filtration apparatus migrated through this gap to the outer diameter of the seal interacting with the spring. Ultimately, the seal degrades and subsequently fails. Therefore, there is a need for a low maintenance, low wear sealing device which can seal the slide plate at low and high polymer pressures and temperatures, particularly for molten materials having low viscosities. The present invention is a filtration apparatus with such a sealing device and, accordingly, solves the above-described problem.